A new class of X-ray optic will be developed that can be used with conventional mammography X-ray tubes, in place of K-edge filters, in order to produce a mono-energetic diagnostic X-ray beam. The X-ray optic is based on proven, currently-available technology, comprising planar reflective multilayer X-ray mirrors that act as extremely efficient energy filters. The multilayer mirror coatings are designed to have a narrow energy bandpass, and can be tuned arbitrarily to specific X-ray energies, for example the Mo Ka line at 17.5 keV, the Rh Ka line at 20.2 keV, or higher energies as desired. The mirror reflection efficiency at the target energy is high, while the reflectance outside of the approximately 0.5 keV bandpass is orders of magnitude smaller. The proposed X-ray optic when used in place of K-edge filters, either in a scanning slot or full-field geometry, and in combination with conventional mammography X-ray tubes, will result in significant increases in both signal contrast and signal-to-noise ratio, while simultaneously reducing patient dose, especially in the case of thicker, more dense breasts, and without any increase in exposure time or degradation of spatial resolution. These expected performance enhancements represent important clinical benefits that will facilitate more accurate and sensitive detection of small lesions, thereby improving the health of a substantional portion of the human population. The specific aims of this Phase I proposal are to fabricate and test a simple, small-field protype X-ray optic of the type just described, and measure the X-ray spectrum it produces in combination with a conventional mammography X-ray tube. Meeting these objectives will demonstrate the technical feasibility of the proposed approach. The long-term (Phases II and III) objectives of this program include a comprehensive experimental study of the clinical performance of these optics, and the development of full-field X-ray optics that can be cost-effectively mass-produced and incorporated into commercial mammography systems.